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Wireless LAN Security I: WEP Overview and Tools. Raj Jain Washington University in Saint Louis Saint Louis, MO 63130 [email protected] Audio/Video recordings of this lecture are available at: http://www.cse.wustl.edu/~jain/cse571-07/. Overview. Wi-Fi Operation

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Wireless lan security i wep overview and tools
Wireless LAN Security I: WEP Overview and Tools

Raj Jain Washington University in Saint LouisSaint Louis, MO [email protected]

Audio/Video recordings of this lecture are available at:



  • Wi-Fi Operation

  • Wired Equivalent Privacy (WEP)

  • Problems with WEP

  • Attack tools

Wi fi operation
Wi-Fi Operation

  • Access Points periodically broadcast a beacon with SSID and security level

  • Subscriber stations listen to these beacons, measure signal strength and determine which AP to join

  • Subscribers can also send a “Probe” to find AP’s in the neighborhood

  • AP authenticates the subscriber station using shared keys

  • Subscriber stations and AP exchange encrypted packets

  • Subscriber station send a “Disassociate” message and log off

Access Point


Mac address filtering
MAC Address Filtering

  • Access Point contains MAC addresses of user NICs

  • Prevents from casual guests logging into the wireless network

  • Problem:

    • Easy to find good MAC addresses by sniffing and then address spoofing

Wired equivalent privacy wep
Wired Equivalent Privacy (WEP)

  • WEP  Privacy similar to a wired network  Intellectual property not exposed to casual browser  Not protect from hacker

  • First encryption standard for wireless. Defined in 802.11b

  • Provides authentication and encryption

  • Shared Key Authentication  Single key is shared by all users and access points

  • Two modes of authentication: Open system and Shared Key

  • Shared Key: Challenge-response verifies client has the key

  • Manual key distribution

  • If an adapter or AP is lost, all devices must be re-keyed

Wep keys
WEP Keys

  • Default Key: Also known as shared key, group key, multicast key, broadcast key. 40-bit or 104 bit. Static.

  • Key mapping key: Also known as individual key, per-station key, unique key. Access points need to keep a table of keys.Not generally implemented.

  • To allow smooth change over, two default keys are required (old and new).

  • WEP allows 4 default keys. Keys are numbered 0..3.  Can use different keys in two directions.

  • Base key is combined with a 24-bit initialization vector (IV)  Different key for each packet

  • WEP does not specify how to select IV. Many vendors generate random IV.

Wep details
WEP Details

  • Each device has 4 static WEP keys

  • 2-bit key ID sent w Initialization Vector (IV) in clear in each packet

  • Per-Packet encryption key =24-bit IV + one of pre-shared key

  • Encryption Algorithm: RC4

    • Standard: 24 + 40 = 64-bit RC4 Key

    • Enhanced: 24 + 104 = 128 bit RC4 key

  • WEP allows IV to be reused

  • CRC-32 = Integrity Check Value (ICV)

  • Data and ICV are encrypted under per-packet encryption key

802.11 Header



802.11 Header




Wep encapsulation
WEP Encapsulation




WEP Keys

K1 K2 K3 K4






MAC hdr




Cipher text


Wep decapsulation
WEP Decapsulation

MAC hdr




Cipher text


WEP Keys

K1 K2 K3 K4














Ron s cipher 4 rc4
Ron's Cipher 4 (RC4)

  • Developed by Ron Rivest in 1987. Trade secret. Leaked 1994.

  • Stream Cipher

    • A pseudo-random stream is generated using a given key and xor'ed with the input

  • Pseudo-random stream is called One-Time pad

  • Key can be 1 to 256 octet

  • See the C code in the textbook [KPS].

Pseudo-random # generator

Encryption Key K

Random byte b

Plain text data byte p

Cipher text data byte c

Wep authentication
WEP Authentication

  • Authentication is a via Challenge response using RC4 with the shared secret key.

Access Point


Challenge (Nonce)

Response (Nonce RC4 encrypted under shared key)

Decrypted nonce OK?

Wep review
WEP Review

  • Four 40-bit or 104-bit Keys are manually programmed in each subscriber station and AP

  • A 24-bit IV and WEP key is used to form a 64b or 128b RC4 key

  • A keystream is generated using the RC4 key

  • A 32-bit CRC is added as “Integrity check value” (ICV) to the packet

  • Plain text and keystream is xor’ed. A 32-bit CRC is added in clear.

Problems with wep authentication
Problems with WEP Authentication

  • Record one challenge/response

  • Both plain text and encrypted text are available to attacker

  • XOR the two to get the keystream

  • Use that keystream and IV to encrypt any subsequent challenges

Access Point


Challenge (Nonce)

Response (Nonce RC4 encrypted under shared key)

Decrypted nonce OK?

Problem with stream cipher
Problem with Stream Cipher

  • Consider two packets with the same IV Þ Same keystream b

  • c1 = p1 b; c2 = p2 b  c1 c2=p1 p2

  • Two packets w same IV  XOR = Difference in plain text

  • 50% chance of using the same IV in 4823 packets.

  • Recovered ICV matches  Plain text is correct

  • Possible to recover all 224 keystreams in a few hours

Problems with wep icv
Problems with WEP ICV

  • CRC is used as ICV

  • CRC: Message polynomial is shifted and divided by CRC polynomial, the remainder is sent as CRC

    p = pnxn + pn–1xn–1 +  + p0x0

  • Remainder(p+q, c) = Remainder(p, c) + Remainder(q, c)

  • ICV is linear: ICV(p+q) = ICV(p) + ICV(q)

  • Conclusion: XOR any CRC-32 valid plain text to encrypted packet. The modified packet will pass the ICV after decryption.

More wep problems
More WEP Problems

  • No centralized key managementManual key distribution  Difficult to change keys

  • Single set of Keys shared by all  Frequent changes necessary

  • No mutual authentication

  • No user management (no use of RADIUS)

  • IV value is too short. Not protected from reuse.

  • Weak integrity check.

  • Directly uses master key

  • No protection against replay

Attack tools
Attack Tools

  • Tools to find wireless networks

  • Tools to monitor traffic

  • Tools to analyze traffic


  • Driving by in a car to find open Wi-Fi networks

  • Based on "War Dialing" to dial all numbers to find modem pools

  • http://Netstumbler.com has on-line database

  • Also, Warstrolling and Warflying

  • Warchalking: Signposting open access points on sidewalk or wall

  • Ref: http://www.warchalking.org

Wardriving tools
Wardriving Tools

  • See http://www.wardriving.com/code.php for a list of 40 wardriving tools


  • Network stumbler, http://netstumbler.com

  • Kismet, http://kismetwireless.net

  • Wellenreiter, http://www.wellenreiter.net

  • Mac Stumbler, www.macstumbler.com for Macs

  • KisMAC, www.binaervarianz.de/projeckte/programmineren/kismac

  • BSD Airtools: A set of free BSD tools for FreeBSD

    • dstumbler for wardriving - w GPS interface

    • www.dachb0den.com/projects/bsd-airtools.html

    • Bootable CD from www.warbsd.com

Network stumbler
Network Stumbler

  • Windows based

  • Records SSIDs and can interface with GPS

  • Ministumbler runs on PDAs and pocket PCs


  • http://kismetwireless.net

  • Linux-base wardriving tool

  • Reads out names of networks as they are discovered (eye-free feature for drivers)

  • Can dump printable strings (may include passwords)

  • List of networks in a CSV file

  • Dump of all packets

  • Dump of packets with weak IV Þ for WEP key finding

Wireless sniffing tools
Wireless Sniffing Tools

Public Domain:

  • Airsnort (Linux / BSD?), http://airsnort.shmoo.com

  • Airosniff (FreeBSD), http://gravitino.net/~bind/code/airosniff/

  • APsniff (Windows), http://www.bretmounet.com/ApSniff/index.asp

  • Aerosol (Windows), http://www.sec33.com/sniph/aerosol.php

  • Mognet (Java/Linux), http://chocobospore.org/mognet/

  • Kismet (Linux), http://www.kismetwireless.net/

  • Prism2Dump (*BSD), http://www.dachb0den.com/projects/bsd-airtools.html

  • Prism Dump, http://developer.axis.com/software/tools/

  • Wellenreiter, http://www.remote-exploit.org/projects.php

Wireless sniffing tools cont
Wireless Sniffing Tools (Cont)

  • wlandump (Linux-WLAN), http://www.linux-wlan.com/

  • WLAN Expert (Windows), http://www.vector.kharkov.ua/download/WLAN/wlanexpert.zip - More of a site survey tool


  • Airopeek, http://www.wildpackets.com/products/airopeek

  • AP Scanner (Mac), http://homepage.mac.com/typexi/Personal1.html

  • Grasshopper, http://www.bvsystems.com/Products/WLAN/Grasshopper/grasshopper.htm - handheld wireless receiver

  • Sniffer Wireless, www.networkgeneral.com

  • Wireless Snif, www.ufasoft.com/sniffer/

    More tools athttp://www.wi-foo.com/index-3.html

Packet analyzers
Packet Analyzers

  • Tcpdump, http://www.tcpdump.org/ , command-line network analyzer for UNIX

  • windump, http://www.winpcap.org/windump/ , Windows version of tcpdump

  • dSniff, http://www.monkey.org/~dugsong/dsniff/ , captures passwords

  • omnipeek, http://www.wildpackets.com/ , packet analysis platform with plugin API

  • snoop, http://en.wikipedia.org/wiki/Snoop_%28software%29 , command-line packet sniffer for Solaris

  • Wireshark (aka Ethereal) (Linux or FreeBSD), http://www.wireshark.org

  • Ngrep, http://ngrep.sourceforge.net/ -string matching in network traffic


  • WEP uses RC4 stream cipher with a fixed set of keysÞ Plain text is xor’ed with a keystream

  • Authentication challenge is sent in clear Þ getting keystream is trivial

  • CRC is used for integrity Þ Easy to modify

  • Plenty of tools to find WiFi APs, monitor and analyze traffic

  • Process of finding open APs is called Wardriving


  • J. Edney and W.A. Arbaugh, “Real 802.11 Security: Wi-Fi Protected Access and 802.11i,” Addison-Wesley, 2004, 481 pp., ISBN:0321156209

  • Krishna Shankar, et al, "Cisco Wireless LAN Security," Cisco Press, 2005, 420 pp, ISBN:1587051540

  • A. A. Vladimirov, K.V. Gavrilenko, and A.A. Mikhailovsky, “Wi-Foo: The Secrets of Wireless Hacking,” Addison-Wesley, 2004, 560 pp., ISBN:0321202171